1. Field of the Invention
The present invention relates to an apparatus and method for performing Hybrid Automatic Repeat reQuest (HARQ) in a broadband wireless communication system. More particularly, the present invention relates to an apparatus and method for determining whether a Mobile Station (MS) receives control information when HARQ is performed and, upon detecting a failure in receiving the control information, for recovering the failure in the broadband wireless communication system.
2. Description of the Related Art
Many wireless communication techniques are being proposed to achieve high-speed mobile communication. Among them, an Orthogonal Frequency Division Multiplexing (OFDM) scheme is being accepted as one of the more promising techniques for a next generation wireless communication. The OFDM scheme is expected to be widely used in future wireless communication systems, and is currently being considered in the Institute of Electrical and Electronics Engineers (IEEE) 802.16 standard, on which a Wireless Metropolitan Area Network (WMAN) system, known as the 3.5 generation technology, is based.
The OFDM scheme uses multi-carriers for data transmission. That is, the OFDM scheme is one type of Multi Carrier Modulation (MCM) scheme in which a serially input symbol stream is converted into parallel symbols and the parallel symbols are transmitted by being modulated into a plurality of orthogonal sub-carriers, i.e., a plurality of sub-channels.
There has been proposed a method in which radio resources are persistently allocated to a Mobile Station (MS) in a broadband wireless communication system based on the OFDM. The persistent resource allocation method is a technique for decreasing an overhead generated when resource allocation information (or a resource allocation message) is transmitted to the MS in every frame (or sub-frame). That is, a Base Station (BS) persistently allocates a specific resource to an MS for which a service (e.g., a Voice over Internet Protocol (VoIP) service, etc.) periodically generating traffic is provided. Thereafter, the MS performs communication by continuously using the persistently allocated resources without having to evaluate the resource allocation information until the persistently allocated resource is changed or released. In this case, the BS does not have to transmit the resource allocation information for the MS in every frame.
Meanwhile, to improve reliability of data transmission, most communication systems use a Hybrid Automatic Repeat reQuest (HARQ) scheme in which a Forward Error Correction (FEC) scheme and an Automatic Repeat reQuest (ARQ) scheme are properly combined. The HARQ scheme corrects an error from a received packet by using an error correction code, and uses an error detection code (e.g., a Cyclic Redundancy Check (CRC) code) of the error corrected packet to detect a retransmission request. In addition, upon receiving a retransmission packet, decoding is performed by combining a currently received packet with a previously received packet. As a result, an additional gain (e.g., a coding gain and an increased Signal to Noise Ratio (SNR)) may be obtained.
As described above, a connection established using HARQ may lead to a problem in which retransmission occurs continuously if link quality is poor. To address this problem, there is ongoing research to persistently allocate resources in the HARQ scheme. Hereinafter, a scheme in which the HARQ scheme and the persistent resource allocation scheme are combined is defined as a “synchronous HARQ” scheme.
The synchronous HARQ scheme is a scheme in which resources of a response signal and a retransmission packet are persistently allocated according to resources used in a 1st transmission. That is, when using the synchronous HARQ scheme, a BS allocates resources only for the 1st transmission, and does not allocate additional resources for the response signal and the retransmission packet. In this case, the BS does not need to transmit allocation information regarding the resources for the retransmission packet and the response signal to the MS. However, if resource allocation information of the 1st transmission may not be received, a receiving end may not receive subsequent retransmission packets.
Further, a position of a resource used for retransmission will inevitably be changed. In this case, the changed resource position should be reported to a corresponding MS. A serious problem may occur if the MS fails to receive resource change information.
For example, for DownLink (DL) transmission, upon detecting a failure in receiving the resource change information, the MS decodes a signal by using a packet at an incorrect resource position (i.e., garbage combining). In this case, the MS may not be able to recover the signal, or may not be able to receive all retransmission packets generated afterwards. For UpLink (UL) transmission, when the MS transmits a UL packet at an incorrect resource position, the BS cannot recover a signal of the MS. In addition, the UL packet transmitted at the incorrect position acts as fatal interference to another MS.
Consequently, if the MS fails to receive control information, there is an increase in a residual packet error rate that is defined as a probability that data reception remains impossible even after a maximum number of retransmissions are performed. This acts as a factor for deteriorating an overall system throughput. The same problem occurs not only in the synchronous HARQ but also in normal HARQ (or asynchronous HARQ).